Valve guides are press fit into smaller diameter cylindrical mating bores contained in internal combustion engine heads to obtain a stable fit between the two components. A stable fit is desirable to prevent subsequent loosening during operation of the engine and to facilitate heat transfer between the components. Thus, it may be appreciated that end-to-end uniformity in density and mechanical properties, such as hardness, of a valve guide is a desirable characteristic. Clearly, increased uniformity leads to improved installation and operation of valve guides.
Blanks for subsequent fabrication into valve guides for use in internal combustion engines have been formed by powder metallurgy processes or have been formed from cast or wrought metal. The present invention relates to an improved powder metallurgy process for making valve guide blanks having improved uniformity in properties.
Powder metallurgy, in general, involves producing metal powders and then utilizing such powders to make desired products. Powder metallurgy processes generally comprise heating a compact formed by a compression of powder metal. Such heating is termed “sintering” and usually does not involve fusion, or at least a major amount of fusion.
Prior powder metallurgy processes for making valve guides generally involve placing metal powders within elongated, cylindrical molds, optionally containing a core rod; applying pressure to the powder through pressure transmission from the top and/or the bottom of the mold (hereinafter referred to as “uniaxial end pressing”) to cause compaction of the powder into the shape of a hollow cylinder; heating the compacted powder to an elevated sintering temperature to cause the compacted powder to bond and form a hollow, cylindrical sintered blank; and then forming the elongated surface of the blank to produce a valve guide. A shape limitation results from the necessity of applying pressure from the top and bottom of the mold, rather than from the side. During the application of pressure, almost no lateral flow of powder is created; and accordingly, desired shapes are limited by such flow characteristic. Holes parallel to the longitudinal axis of the mold can readily be made with the use of a core rod, if desired, but holes perpendicular to such axis must be machined following compacting and sintering. Uniaxial side pressing thus precludes the use of a core rod to form a hollow center in the blank because such hollow center would be perpendicular to the longitudinal axis of the mold. A typical prior art powder metallurgy process for making valve guides is illustrated in U.S. Pat. No. 6,599,345.
The above-described uniaxial end pressing of the prior art involves significant limitations because metal powders tend to cling to the sides of the mold and thereby result in unequal pressure throughout the body of powder metal. Thus, parts formed by applying pressure at both ends of the mold are usually less dense in the middle of the resultant hollow sintered blank. Thus, non-uniform density and non-uniform mechanical properties are created in the blank and resultant valve guide. Metal powders under pressure do not flow like fluids. The powders do not tend to flow, for example, from a region of high compression to one of low compression; and as a consequence, the depth-width ratio is extremely important in the dies used in the process. The manufacture of compacts of about four inches in length or greater becomes very difficult because of pressing force limitations associated with commercial pressing operations. Similar pressing force limitations are encountered when attempting to press compact diameters of about ⅝ inches or more. Consequently, the above-described uniaxial end pressing process has major limitations when relatively large valve guides are desired.
The above-mentioned limitations of the prior art are addressed and improved by the process and product of the present invention in a straightforward and effective manner. Rather than applying compacting pressure in a direction parallel to the longitudinal axis of the mold (uniaxial end pressing), compacting pressure is applied solely in a side direction substantially perpendicular to the longitudinal axis of the metal powder filled mold (uniaxial side pressing). A pressing apparatus may be used to apply pressure in one side direction only or in two opposed side directions. Uniaxial side pressing eliminates lower density at the mid-length of the pressed compact that is characteristic of uniaxial end pressing because pressing forces are exerted from a different direction, and the length of material in the pressing direction is sufficiently short so as to not encounter substantial density and hardness variation throughout the cross-section of the pressed compact. Obviously, end-to-end variation in density and hardness is minimized by the process of the invention because pressure is not applied to the ends of the mold. In addition, there is no problem in making compacts of about four inches or more in length or compacts having diameters of about ⅝ inches or more. Uniaxial side pressing involves another advantage because it becomes a routine procedure to make compacts of a master length and then cut such compacts into desired blank lengths for subsequent processing into valve guides. Such master length process thereby reduces the number of pressing and sintering operations required to manufacture a given number of valve guides.
Applicant does not believe that it has invented uniaxial end pressing in a general sense because such technique is found in the art. Please see U.S. Pat. Nos. 4,976,915; 6,080,358; and 6,221,813 in this regard. However, Applicant believes that its contribution to the art lies in advantageously adapting such technique to the manufacture of valve guides in a manner having significant advantages when contracted with prior valve guide manufacturing processes.
Above-mentioned U.S. Pat. No. 6,599,345 discloses at column 5, lines 36 and 37 that compaction can be performed either uniaxially or isostatically. No further explanation is offered to expressly define the meaning of uniaxial pressing. However, the disclosure at column 6, lines 13-58 makes it apparent that uniaxial end pressing, rather than uniaxial side pressing, is contemplated by the patent. Applicant believes that the above conclusion is supported by the presence of an inner bore in the intermediate product prior to forming the valve guide. Such presence of an inner bore means that the bore must have been created during the compaction step and further that uniaxial end pressing would necessarily have been applied. One would not utilize uniaxial side pressing to make a hollow centered intermediate product because valve guides require exact centering and circular shape due to the intended use and function in an internal combustion engine. Uniaxial side pressing does not lead to sufficient specificity as to where the center will ultimately be located in the final product to warrant an attempt to produce a hollowed blank. Thus, in the present invention, following uniaxial side pressing and sintering, a solid blank is produced; formed into the desired circular shape; and a center hole then formed. When uniaxial end pressing is utilized, the intended center can be determined more exactly because the applied pressure is parallel, rather than perpendicular, to the longitudinal axis of the mold. Thus, a hollow blank may be formed at the compaction stage. In any event, the process of the invention utilizes uniaxial side pressing to produce a solid—not hollowed—intermediate blank product which is later formed into a valve guide by forming into a round shape and then forming an open central portion in the intermediate product. It is evident that the respective valve guide manufacturing processes are quite distinct.